The induction of T-lymphocyte activation requires one signal from an exogenous agent such as an antigen plant lectin or allogeneic cell and second signal from a macrophage. Phorbol myristic acetate (PMA) enables macrophage depleted T cells to proliferate in response to a mitogenic but not an antigenic stimulus. Since the macrophage signal is replaced by PMA, we can more readily study the other signal required for activation. We have demonstrated that culture supernatants of peritoneal exudate lymphocytes (PELS) pulsed with an antigen have mitogenic activity only in the presence of PMA. This factor has the following characteristics. There is a restriction for antigen specificity at the production end of this T-cell mitogenic factor (TMF) since supernatants of PELS have mitogenic activity only when pulsed with an antigen with which the donor guinea pig has been primed. There appears to be no restriction for antigen specificity at the effector end of this TMF activity since an active supernatant is able to induce mitogenic activity on PMA treated T cells of unimmunized animals. We plan to further characterize the nature of this factor, which resembles T cell growth factor and will begin the biochemical characterization of this factor.